Cassini – a solar system grand finale

As NASA’s Cassini spacecraft spends its last few weeks in orbit around Saturn before making a controlled impact with the planet in what NASA dubbed Cassini’s “grand finale,” scientists and engineers who helped launch the mission 20 years ago are thrilled with the success of the massive probe they helped dispatch to one of the solar system’s most intriguing worlds.

It took seven years for Cassini to make the trip from Earth’s surface to Saturn’s orbit. The two-story-tall spacecraft reached Saturn on 30 June 2004, to begin unprecedented surveys of the planet and its brilliant rings, along with its eclectic system of moons.

The spacecraft began its finale on 26 April 2017 and will continue its dives until 15 September 2017when it will make a mission-ending plunge into Saturn’s atmosphere to prevent it from accidentally contaminating one of the moons if it were left to drift around in space.

Why Cassini?

NASA’s Cassini spacecraft and ESA’s Huygens probe expanded scientists’ understanding of the kinds of worlds where life might exist. With discoveries on Saturn’s moons Enceladus and Titan, Cassini and Huygens made exploring “ocean worlds” a major focus of planetary science. Insights from the mission also helped to look for potentially habitable planets – and moons – beyond our solar system.

In December 18, 2016, when Cassini was traveling through the outer edge of the Saturn’s rings, the RPWS data showed that Cassini had suffered hundreds of thousands of dust particles. The second figure is on 26 April, 2017, with Cassini and Saturn from the inner side of Saturn’s body through the gap, and RPWS shows almost no record of dust particles’collision event. Source: NASA\/JPL-Caltech\/University of Iowa.

Life as we know it is thought to be possible in stable environments that offer liquid water, essential chemical elements, and a source of energy (from sunlight or chemical reactions). Before Cassini launched in 1997, it wasn’t clear that any place in the icy outer solar system (that is, beyond Mars) might have this mix of ingredients. By the next year, NASA’s Galileo mission revealed that Jupiter’s moon Europa likely has a global ocean that could be habitable. Since its 2004 arrival at Saturn, Cassini has shown that Europa isn’t an oddball: ptentially habitable ocean worlds exist even in the Saturn system – ten times farther from the sun than Earth.

When the Cassini mission started, scientists presumed Enceladus was too small to generate and hold onto the heat required to maintain subsurface reservoirs of liquid water. Cassini’s discovery of intense geologic activity near the moon’s unexpectedly warm South Pole – complete with towering jets of icy spray — sent shockwaves through the space science community. After over a decade of investigation, the mission eventually determined that Enceladus hosts a global liquid water ocean, with salts and simple organic molecules, and likely even hydrothermal vents on its seafloor. Thanks to Cassini, Enceladus is now one of the most promising places in our solar system to search for present-day life beyond Earth.

Saturn’s largest moon, Titan, offered tantalising hints that it, too, could help us understand whether life could have evolved elsewhere. Cassini and ESA’s Huygens probe (which landed on Titan’s surface) found clear evidence of a global ocean of water beneath Titan’s thick, icy crust and an atmosphere teeming with prebiotic chemicals. Based on modelling studies, some researchers think Titan, too, may have hydrothermal chemistry in its ocean that could provide energy for life. On its frigid surface, which hosts vast seas of liquid hydrocarbons, scientists wonder whether Titan could be home to exotic forms of life “as we don’t know it”?

On Saturn’s largest moon, Titan, Cassini and Huygens showed one of the most Earth-like worlds ever encountered, with weather, climate and geology that provide new ways to understand our home planet.

Cassini is, in a sense, a time machine. It has given a portal to see the physical processes that likely shaped the development of our solar system, as well as planetary systems around other stars.

The length of Cassini’s mission has enabled scientists to observe weather and seasonal changes, improving understanding of similar processes on Earth, and potentially those on planets around other stars.

Cassini revealed Saturn’s moons to be unique worlds with their own stories to tell. It also showed the complexity of Saturn’s rings and the dramatic processes operating within them.

Some of Cassini’s best discoveries were serendipitous. What Cassini found at Saturn prompted scientists to rethink their understanding of the solar system. This mission represents a staggering achievement of human and technical complexity, finding innovative ways to use the spacecraft and its instruments, and paving the way for future missions to explore our solar system.

Early magnetic field analysis at odds with scientists

Based on data collected by Cassini’s magnetometer instrument, Saturn’s magnetic field appears to be surprisingly well-aligned with the planet’s rotation axis. The tilt is much smaller than 0,060 – which is the lower limit the spacecraft’s magnetometer data placed on the value prior to the start of the grand finale.

This observation is at odds with scientists’ theoretical understanding of how magnetic fields are generated. Planetary magnetic fields are understood to require some degree of tilt to sustain currents flowing through the liquid metal deep inside the planets (in Saturn’s case, thought to be liquid metallic hydrogen). With no tilt, the currents would eventually subside and the field would disappear.

Any tilt to the magnetic field would make the daily wobble of the planet’s deep interior observable, thus revealing the true length of Saturn’s day, which has so far proven elusive.

This zoomed-in view of Epimetheus, one of the highest resolution ever taken, shows a surface covered in craters, vivid reminders of the hazards of space.

The lack of a tilt may eventually be rectified with further data. Scientists believe some aspect of the planet’s deep atmosphere might be masking the true internal magnetic field. The team will continue to collect and analyse data for the remainder of the mission, including during the final plunge into Saturn.

The magnetometer data will also be evaluated in concert with Cassini’s measurements of Saturn’s gravity field collected during the grand finale. Early analysis of the gravity data collected so far shows discrepancies compared with parts of the leading models of Saturn’s interior, suggesting something unexpected about the planet’s structure is awaiting discovery.

In addition to its investigation of the planet’s interior, Cassini has now obtained the first-ever samples of the planet’s atmosphere and main rings, which promise new insights about their composition and structure. The spacecraft’s cosmic dust analyser (CDA) instrument has collected many nanometre-size ring particles while flying through the planet-ring gap, while its ion and neutral mass spectrometer (INMS) has sniffed the outermost atmosphere, called the exosphere.

During Cassini’s first dive through the gap in April, the spacecraft was oriented so that its large, saucer-shaped antenna would act as a shield against oncoming ring particles that might cause damage. While at first it appeared that there were essentially no particles in the gap, scientists later determined the particles there are very small and could be detected using the cosmic dust Analyser (CDA) instrument.

The CDA was later allowed to peek out from behind the antenna during Cassini’s third of four passes through the innermost of Saturn’s main rings, the D ring, on 29 June 2017. During Cassini’s first two passes through the inner D ring, the particle environment was found to be benign. This prompted mission controllers to relax the shielding requirement for one orbit, in hope of capturing ring particles there using CDA. As the spacecraft passed through the ring, the CDA instrument successfully captured some of the tiniest particles, which the team expects will provide significant information about their composition.

During the spacecraft’s final five orbits, as well as it final plunge, the INMS instrument will obtain samples deeper down in the atmosphere. Cassini will skim through the outer atmosphere during these passes, and INMS is expected to send particularly important data on the composition of Saturn’s atmosphere during the final plunge.

Cassini, which was built and operated from the Jet Propulsion Laboratory in Pasadena, California, lifted off from Cape Canaveral, Florida, in 1997 atop a Titan IVB rocket. The launch capped months of processing on the spacecraft and booster to make sure everything would work correctly on launch day and Cassini would be set on its correct path.

Despite the size of the rocket, Cassini still took seven years to reach Saturn. It flew a precise path through the inner solar system building up speed by passing near Venus and Earth so it could slingshot out beyond Mars and Jupiter to rendezvous with Saturn. Getting on that path correctly at the start was the focus of the launch team.

Cassini also carried a second spacecraft along with it, a probe called Huygens that was released to study the moon Titan up close. Huygens parachuted to the surface of the moon, relaying data on conditions there until its batteries ran out.

Fig. 1 shows the view of Saturn’s moon Epimetheus (116 kilometres) captured by NASA’s Cassini spacecraft during a moderately close flyby on 6 December 2015. This is one of Cassini’s highest resolution views of the small moon, looking toward the Saturn-facing side of Epimetheus. North on Epimetheus is up. The image was taken in green polarised light with the Cassini spacecraft narrow-angle camera. The view was obtained at a distance of approximately 35,000 km from Epimetheus and at a sun-Epimetheus-spacecraft, or phase, angle of 280.

While Cassini’s mission will end on 15 September 2017, the data captured will keep scientists busy for many years to come.